Remove unwanted material
To find all the objects currently using a material.


If you have one of the objects selected,  ShiftL (Select Linked) > Material will select all visible objects using the same material as the one in the active slot. ( you may have to unhide objects to select them).
If you can't easily track down the object, you can switch the Outliner to Data API view (highlighted)..




.. select the material you're interested in, right-click, and, again, Select Linked.

To unlink a material from all objects ..
(Setting users to 0, even if you haven't found them)


Shift-click the 'Unlink' button in the materials panel ..


..Although it will continue to appear in the dropdown, the material will not be saved in the .blend file. Next time you open the file, the material will be gone.